1. Field of the Invention
The present invention relates to a polyester packaging material. More particularly, the present invention relates to a polyester packaging material with excellent gas barrier properties, the mechanical strength and transparency.
2. Description of the Related Art
Polyethylene terephthalate has excellent mechanical and chemical properties and therefore, it is widely used for the production of fibers, films and industrial resins, and recently, polyethylene terephthalate is used for the production of various vessels such as bottles, cups and trays. In this application field, in view of the storage of the contents, a high gas barrier property is required. Polyethylene terephthalate is excellent in the gas barrier property over polyolefin resins such as polyethylene, but as compared with the gas barrier property of glass or aluminum, the gas barrier property of polyethylene terephthalate is still insufficient.
Investigations have been made for improving the gas barrier property of polyethylene terephthalate, and various methods have been proposed. For example, there have been proposed a method in which a polyester is coated or laminated with a gas barrier material such as polyvinylidene chloride or a saponified ethylene/vinyl acetate copolymer (see Japanese Unexamined Patent Publications No. 54-117565 and No. 56-64839), a method in which a polyester is blended with a gas barrier material (see Japanese Unexamined Patent Publication No. 57-10640 and No. 59-196316 and European Patent Application Publication No. 0105825) and a method in which the gas barrier property is improved by increasing the orientation degree of a polyester shaped article (see Japanese Unexamined Patent Publication No. 56-151648). In the method of increasing the orientation degree, however, the improvement of the gas barrier property is limited, and the method using a gas barrier material involves problems as regards recovery and re-utilization of the used articles. In the method in which a polyester is coated or laminated with a gas barrier material, since a resin different from the polyester is used, the adhesion of the resin to the polyester is poor and interlaminar peeling or reduction of the transparency occurs. Moreover, the method is disadvantageous in recovery of the used material. When polyethylene terephthalate is laminated or coated with polyethylene isophthalate, (see Japanese Unexamined Patent Publication No. 59-39547 and European Patent Application Publication No. 0105825), because of the brittleness of polyethylene isophthalate per se, the laminated or coated structure is brittle and in order to obtain a strength required for a vessel, the thickness of the polyethylene terephthalate layer should be increased to a level corresponding to the thickness of a vessel composed solely of a polyethylene terephthalate layer, and therefore, the weight of the vessel as a whole is increased and a light weight characteristic, which is one of merits of synthetic resin vessels, is lost. Furthermore, the gas barrier property is poor. In the case of a vessel, since the heat resistance of a polyethylene terephthalate packing material laminated with a polyglycolic acid (see U.S. Pat. No. 4,424,242) is poor, it is difficult to obtain a molded article having a satisfactory mechanical strength under molding conditions for polyethylene terephthalate. Further, a polyester packing material of polyalkylene isophthalate copolymerized with an aliphatic dicarboxylic acid having 4 to 12 carbon atoms are proposed (see U.S. Pat. No. 4,403,090). However, this material does not have a satisfactorily improved gas barrier property.